Luminescence is photon emission initiated by energy forms other than thermal agitation. Phosphorescence denotes luminescence of long-duration. The ability of a phosphorescent solid to emit photons upon excitation is dependent upon the arrangement and population of the electronic energy levels of its constituent molecules. Electrons situated in metastable states migrate to higher energy levels under excitation, and emit photons upon their return to the lower levels. The requisite electronic arrangement is achieved by incorporation of a foreign ion, or inorganic activator in a host crystal, and the combination is termed a luminescent center.
Chemical interaction between a luminescent center and another chemical entity may adversely affect the desired electronic arrangement, causing decreased luminescence. Iron, cobalt, potassium, and arsenic are such entities, and are thereby called poisons. The synergistic electronic band configuration between host and activator is also disturbed by physical treatment such as grinding and excessive heating. At present, inventions comprising phosphor-glass compositions generally involve manufacturing processes for glass films not more than 5 mills in thickness. Said films are deposited on a substrate to form screens which are used in the manufacture of cathode-ray tubes, electro luminescent devices, fluorescent lighting, luminous markers, dials and indicators, and radiation detection and measurement instruments.
Crystalline luminescent ceramic materials, where the phosphor crystals are formed in situ from glasses, can be used to form luminescent artifacts of arbitrary physical dimensions. However, the luminescence produced by such a material is of short duration. Such materials are primarily applied in thin films for photo and electroleminescent devices. Heretofore, serious difficulties have been encountered in the process of incorporating phosphors into a glass matrix because the phosphors cannot readily withstand the high temperatures of the molten or softened glass, and are deleteriously affected or destroyed by the excessive heat even in a relatively short time. As a result, glass products containing phosphors show a substantial loss in light output efficiency.
Fusion of a phosphor-glass frit mixture at lower temperatures is not desirable due to air becoming trapped in the mixture as fusion takes place. This results in many of the phosphor particles failing to become completely encapsulated by the glass. The structural properties of such a fused mix are rather poor.